Mechanism of persistence of MARS-containing pEPI-1 vectors. (A) Episomal replication can be explained by a stable association with early replication foci mediated by the MARS mediated binding to the nuclear matrix protein scaffold attachment factor A (SAF-A). Within early replication foci, the assembly of the origin recognition complex (ORC) and the DNA replication of pEPI vector molecules is likely facilitated by a conformational change resulting from mRNA transcription.(B) Nuclear retention and mitotic stability of pEPI-based vectors can be explained by MARS mediated direct or indirect interaction with metaphase chromosomes in a "piggy back"-like mechanism. (C) The functional element of pEPI-vector plasmid replicons consists of a transcription unit, regulated by a constitutive promoter and directed into a chromosomal MARS with the prerequisite of no termination signal being located between transcription unit and MARS element.